Colloidal TiO₂ Catalyzed Photo‐ and Radiation Chemical Processes in Aqueous Solution

A number of free radical reactions was observed upon the illumination of TiO 2 sols with light of the near UV. Cl − , Br − , and I − were oxidized to yield the radical anions Cl − 2 , Br − 2 , and I − 2 , respectively. Propanol‐2 was oxidized to yield the 1‐hydroxy‐1‐methyl ethyl radical. These intermediates were identified by their optical absorptions or their reaction with tetranitromethane or acrylonitrile. The quantum yields were measured as a function of the substrate concentration. The effects are explained by the formation of electron‐hole pairs in illuminated colloidal TiO 2 , rapid diffusion of these charge carriers to the surface of the colloidal particles, and reaction with adsorbed acceptors. Most of the charge carriers, however, disappeared by recombination, the largest quantum yield observed being 0.16. The lifetime of the charge carriers was less than 30 ns.—Acidic TiO 2 sols containing propanol‐2 were also γ‐irradiated. In these sols, 1‐hydroxy‐1‐methyl ethyl radicals were formed which attacked the colloidal particles. The overall reaction consisted of the dissolution of the colloid. At short irradiation times, the sols became blue, and this color slowly disappeared upon further irradiation. The color centers had an absorption coefficient of 1 · 10 4 M −1 cm −1 . They were very sensitive towards oxygen. Their formation is connected with the electron transfer from the organic radicals to the colloidal particles. A mechanism is proposed according to which the energy of dissolution of colloidal TiO 2 is supplied by the overall disproportionation of the radicals.